1. Field of the Invention
The present invention relates to an air-cleaning photocatalytic filter utilizing a photocatalyst capable of cleaning air by utilizing a photoreaction. More particularly, the present invention relates to an air-cleaning photocatalytic filter which is used as a filter of an air-cleaning apparatus arranged in a place exposing to natural or artificial light in a vehicle, train, ship or house, to clean the ambient air to, for example, deodorize the air.
2. Description of the Related Art
Currently, many attempts are made to utilize a photocatalyst which realizes various functions, for example, deodorization, antibacterial function and stain-proofing, upon being irradiated by the light, for the purpose of cleaning the environment.
As an air-cleaning photocatalytic method, a method in which odorous substances, for example, aldehyde compounds, in air are decomposed and removed and/or NOx gas is oxidized to convert it to nitric acid and is removed from the air by using a titanium dioxide photocatalyst, is advantageous in that no electric energy and no specific apparatus are necessary and thus the study of the practical utilization of the method has been started.
In a practical utilization of the photocatalyst, for example, Japanese Unexamined Patent Publication No. 7-171,408 discloses a utilization of a composite material having a support material and a coating layer formed on a surface of the support material and comprising a mixture of photocatalyst particles and a binder having a high resistance to photodecomposition.
In this type of the coating layer, however, since the photocatalyst particles, per se, exhibit an air-cleaning function upon absorbing ultraviolet rays therein, when the photocatalyst particle-containing coating layer is formed in a large thickness on the support material surface, only the portion of the photocatalyst particles which are located in a surface portion of the coating layer and are exposed to the irradiation of the ultraviolet rays, can fulfil the photocatalytic function. Namely, the other portion of the photocatalyst particles which are located inside of the coating layer and to which the ultraviolet rays do not reach, does not work as a photocatalyst. Thus, the photocatalyst particles contained in the coating layer exhibit an insufficient reaction efficiency.
The above-mentioned problem forms an obstructive barrier to the practical utilization of the photocatalyst particles for use in, for example, air-cleaning and water-purification, for which a high reaction rate and a high reaction efficiency are required.
As a means for solving the above-mentioned problem, for example, Japanese Unexamined Patent Publication No. 8-71573 discloses a photocatalytic method in which a light-transmissible material having a roughened surface is used as a support, and photocatalytic particles are supported on the roughened surface of the support, and thus the resultant photocatalytic coating layer has an increased surface area.
Also, Japanese Unexamined Patent Publication No. 7-171,408 discloses a photocatalytic coating layer formed from a coating liquid prepared by mixing photocatalyst particles with a binder comprising water glass and/or a silicone polymer. In this coating layer, however, the water glass and/or the silicone polymer which serves as a binder and exhibits no effect on enhancement of the photocatalytic activity of the photocatalyst particles, covers the surfaces of the photocatalyst particles and thus usually disadvantageously causes the photocatalytic activity of the photocatalyst particles to be reduced.
As mentioned above, no means for causing the all of the photocatalyst particles contained in the photocatalytic coating layer to fully function, by controlling the compositions of the coating liquid and the coating layer has been found and thus the function of the conventional photocatalytic coating layer has a limit.
Further, the above-mentioned coating layer is disadvantageous in that the coating liquid containing an alkaline material, for example, a water glass (sodium silicate) causes the support material to be corroded and the coating operation circumstances to be degraded. Also, when the coating layer contains a silicone polymer, since a catalyst, for example, an acid, must be used and/or the silicon polymer must be used together with a large amount of an organic solvent, the coating operation circumstances are deteriorated. Thus, the above-mentioned conventional photocatalytic coating layer must be further improved.
An object of the present invention is to provide an air-cleaning photocatalytic filter which is provided with a surface formed by a photocatalytic coating layer formed on a support material with a high adhesion and having an excellent durability, exhibits a high efficiency on air-cleaning including deodorization and removal of nitrogen oxides from the air, and thus is free from the disadvantageous of the conventional photocatalytic coating layer and air-cleaning photocatalytic filter, namely, a low efficiency of photocatalytic activity.
The above-mentioned object can be attained by the air-cleaning photo-catalytic filter of the present invention which comprises:
a base material, and
a coating layer formed on a surface of the base material, comprising 5 to 60% by weight of ultraviolet ray-transmissible solid particles having a minor axis of the particles in the range of from 0.2 to 50 xcexcm, 20 to 80% by weight of photocatalytic solid particles having an average particle size of 0.001 to 0.02 xcexcm and 10 to 60% by weight of fine silica particles having an average particle size of 0.002 to 0.2 xcexcm. and having a thickness of 5 to 60 xcexcm.
In the air-cleaning photocatalytic filter of the present invention, the coating layer optionally further comprises 0.2 to 20% by weight of a clay mineral material.
In the air-cleaning photocatalytic filter of the present invention, at least a portion of the ultraviolet ray-transmissible solid particles located in the surface portion of the coating layer is preferably exposed to the outside of the photocatalytic filter.
In the air-cleaning photocatalytic filter of the present invention, the ultraviolet ray transmissible solid particles are preferably selected from the group consisting of pulverized glass particles and glass fibers.
In the air-cleaning photocatalytic filter of the present invention, the minor axis of the ultraviolet ray-transmissible solid particles is preferably in the range of from 0.1 to 1.5 times the thickness of the coating layer.
In the air-cleaning photocatalytic filter of the present invention, the base material preferably comprises a member selected from the group consisting of aluminum materials, paper sheets ceramic fiber nonwoven fabrics, and plastic resin materials.
In the air-cleaning photocatalytic filter of the present invention, the base material is preferably in the form of a panel, sheet or honeycomb.
In the air cleaning photocatalytic filter of the present invention, the photocatalytic solid particles are preferably selected from the group consisting of titanium dioxide particles, zinc oxide particles, strontium titanate particles and tin dioxide particles.
In the air-cleaning photocatalytic filter of the present invention, the clay mineral material preferably comprises at least a member selected from the group consisting of natural and artificial smectite, sepiolite, talc, swelling mica and kaolin.